The present invention relates to electric power plants and more particularly to flow controls for once-through boilers.
It has been common to employ an off-stream flash tank for startup of a once-through boiler. For example, the boiler may be designed to provide 246.05 KGSC (kilograms per square centimeter) steam at the turbine during normal operation and the flash tank may be designed to supply steam at 70.3 KSGC for startup and operation of the turbine up to about 10% load. During all startups, a conventional V valve between the primary and secondary superheater is closed, and conventional P and N valves are open. Water is heated in furnace section and primary superheater passes and flows into the flash tank where the steam is separated from the water. The steam then flows back to the final superheater where additional heat is added before flowing to the turbine. The steam generated in this manner requires coordinated action to open the V valve, close a P valve, and modulate the firing rate and adjust the turbine valves to avoid interaction which could cause abrupt changes in steam temperature or pressure or turbine/generator output. Operation with this type system has been satisfactory for base loaded units. However, the complex startup and shutdown procedure has made it unsuitable for cycling service.
More recently, an integral separator starting system has been developed for Foster-Wheeler once-through boilers, and it decreases the complexity of starting and increases the availability and cycling capacity of the once-through boiler. In this arrangement, the low pressure off-line flash tank is eliminated and a set of separators are installed in the main flow path upstream from the primary superheater. In this manner, provision is made for: (1) water to flow through the furnace tubes at a minimum rate to protect them from overheating; (2) diversion of water from the turbine to a drain flow path in which drain valves are provided reaches its normal operating range; and (3) a startup supply of steam until the boiler reaches its normal operating range.
In the integral separator startup system, a plurality of sets of valves are disposed upstream from the separators to control the waterwall pressure or the back pressure on the boiler feed pump as the boiler is ramped into operation. A set of W valves and a parallel set of Y valves have been coordinatively operated in the Foster-Wheeler boiler to control waterwall pressure as the boiler moves up to 25% load. Above 25% load, the W and Y valves are open. An additional set of PPR valves are also provided in parallel with the W and Y valves to provide finer waterwall pressure control over a lower pressure operating range.
During the boiler startup, as the boiler is fired to increase the operating pressure along a ramp, the separator drain valves are operated to produce the water drain flow needed to keep the separator water level at a setpoint value. The boiler inflow and outflow are impliedly balanced by the separator level control. When the boiler reaches a supercritical state, the fluid in the separators in supercritical and the drain valves are closed since the total boiler outflow can then be sent to the turbine without danger of water carryover.
The level control for the separator drain valves acts in response to a level detector which senses steam and water weights and uses the weights to generate a signal indicative of the steam-water interface, i.e. the water level. At 246.05 KGSC, the fluid is supercritical and the level detector is not usable since no steam-water interface exists. However, since the drain valves are closed, there is no need for level control at 246.05 KGSC and above.
In the transition pressure range below 246.05 KGSC pressure, control of the drain valves is needed for boiler flow balance, yet the level detector output is subject to error and therefore is not reliable for level control. In fact, the level measurement is unusable from 210.9 KGSC and up since it will produce a drain valve closure and a boiler flow imbalance, i.e. more inflow than outflow which causes a jump in the waterwall pressure from 210.9 KGSC to 249.57 KGSC, a chilling of the boiler water and a plant trip for turbine protection. There is accordingly a need for an improved control which provides for smooth and reliable startup of once-through boilers having integral separators.